1. Field of the Invention
The present invention relates to a molding material for shell molds and cores and a shell mold made therefrom, and, more particularly, it relates to a molding material which gives off only a small amount of smoke and/or irritating gases (including offensive odors and poisonous gases resulting from the decomposition of resin) and to a mold made therefrom.
2. Description of the Prior Art
Shell molding process has been in general use for the production of molds and cores for casting (collectively called molds hereinafter). According to the shell molding process, sand molds are produced by using as the binder a phenolic resin or other synthetic resin which cures upon heating. Molds produced by this shell molding process provide castings with superior finish and high dimensional accuracy. The molding material used for the shell molding process is resin-coated casting sand, e.g. fine silica sand coated with a thermosetting resin such as phenolic resin.
The resin-coated casting sand, however, has a disadvantage. That is, it gives off irritating gases of formaldehyde, phenol, ammonia, etc. in the coating process and when it is made into molds with heating. Molds of resin-coated sand also give off irritating gases at the time of casting. These irritating gases aggravate the working environment of a foundry and make it necessary to install a costly air cleaner. Further, even such costly air cleaner still does not serve the purpose well.
In order to eliminate this disadvantage, there was proposed a molding material consisting of sand and unsaturated polyester resin as a coating resin. (Japanese Laid-open Pat. Publication Nos. 80234/1976 and 59560/1981.) This molding material gives off a less amount of irritating gases than those using phenolic resin-coated sand. However, it is poor in moldability.
On the other hand, there is a problem where the conventional resin-coated sand is used. That is, molds made from it do not readily disintegrate after casting, especially where the casting temperature is low, as in the casting of aluminum and magnesium. Incomplete mold disintegration leads to additional labor for sand strip and core removal.
In the case of ferrous metal casting, the shell molds easily disintegrate, permitting cores to be removed easily from the casting. This is because the poured molten metal heats the molds up to 800 to 1000.degree. C. at which phenolic resin (or other thermosetting resin) as the binder is pyrolyzed almost completely, and the molds thus naturally lose their strength after casting. However, mold disintegration does not take place readily in the case of aluminum or other low-melting metals. This is because the mold temperature after pouring is 300 to 400.degree. C., at which the binder does not decompose completely, and the molds still retain most of their strength even after casting. This situation make it very difficult to remove cores from casting of complex shape. To facilitate the removal of cores, it is necessary to heat the mold at about 500.degree. C. for a long time in a furnace. This step requires a large amount of energy and lowers productivity.
In order to solve the above-mentioned problems, the present inventors carried out a series of researches, which led to the present invention.